Embodiments of the present specification relate generally to diagnostic imaging, and more particularly to methods and systems for joint estimation of attenuation and activity information.
Positron emission tomography (PET) finds use in generating images that represent a distribution of positron-emitting nuclides, for example, within a patient's body. Accordingly, during PET imaging, a radionuclide is injected into the patient. As the radionuclide decays, positrons are emitted that collide with electrons, thereby resulting in an annihilation event. The annihilation converts the entire mass of the positron-electron pair into two 511 kilo-electron volt (keV) photons emitted in substantially opposite directions along a line of response (LOR). The PET system includes one or more detectors that are placed along the LOR on a detector ring to detect the annihilation photons. Particularly, the detectors detect a coincidence event if the photons arrive and are detected at the detector elements within a coincidence time window. Subsequently, the PET system uses the detected coincidence information along with other acquired image data for ascertaining localized concentrations of the radionuclide for use in generating a functional diagnostic image.
However, during imaging, the photon-electron interactions may result in attenuation of emitted photons, which in turn, may lead to inaccurate PET quantitation and/or degraded image quality. Accordingly, certain PET imaging approaches are drawn to joint estimation of PET attenuation and activity or emission maps from PET emission scan data, where all voxels/pixels are initially unknown. However, the conventional joint estimation approaches may result in cross-talk artifacts and incorrect scaling due to an under-determinedness and ill-conditionedness of the corresponding inverse problem, thus leading to incorrect PET attenuation correction.
Accordingly, PET imaging is often combined with an external radioactive source to measure attenuation factors in projection-space or to reconstruct an attenuation map in image-space that is representative of a spatial distribution of linear attenuation coefficients for the emission photons. Alternatively, an attenuation map is obtained from anatomical scan data acquired using an anatomical imaging scanner. For example, in conventional emission tomography systems, PET imaging may be combined with computed tomography (CT) or magnetic resonance imaging (MRI) to correct for the photon attenuations.
Although CT may produce anatomical transmission data of desired statistical quality, CT imaging provides limited soft-tissue contrast and involves administering substantial radiation to a patient. Accordingly, in certain imaging scenarios, MRI may be used in conjunction with PET imaging for generating high-quality images for use in providing efficient diagnosis and/or treatment to a patient. To that end, MRI and PET scans may be performed sequentially in separate scanners or simultaneously in a combined PET/MRI scanner. Particularly, simultaneous acquisition of PET and MRI data provides unique opportunities to study biochemical processes through fusion of complementary information determined using the orthogonal MRI and PET imaging modalities.
MRI, however, may not provide a direct transformation of magnetic resonance (MR) images into PET attenuation values. Generally, the MR images reflect distribution of hydrogen nuclei with relaxation properties rather than electron density, which is related to PET attenuation. Accordingly, certain conventional imaging approaches employ segmentation or atlas-based registration of the MR images to produce a corresponding patient-specific attenuation map. The attenuation map is then forward-projected to determine attenuation factors, which in turn, are used to reconstruct corresponding PET activity or emission images. Use of MRI information, thus, enhances PET attenuation correction and the subsequent PET image reconstruction.
However, the MRI information provides insufficient distinction between regions including lungs, air, bone, and/or metal even though these constituent materials have substantially different PET attenuation values. Accordingly, use of conventional segmentation and/or atlas-based approaches for estimation of PET attenuation maps using MRI information may result in inaccurate attenuation correction, particularly in and/or near metal, bones, and lungs, subsequently leading to PET activity quantitation errors. Particularly, the atlas-based approaches may be unable to address significant inter-patient variations in anatomy particularly for patient body parts other than heads. Moreover, in certain scenarios, MRI may provide only a truncated field of view (FOV) and may not suitably account for presence of extra-patient components such as beds and coils in the vicinity. The truncated FOV and the extra-patient components may also contribute to photon attenuation, in turn, leading to inaccurate PET quantitation and/or degraded image quality.
Accordingly, there is a need for a method and a system that mitigate the shortcomings of these conventional approaches to provide accurate attenuation correction in emission scan data. Particularly, it would be desirable to design a method and a system that address insufficiency and/or inaccuracy of MR-based attenuation images to provide efficient estimation of the attenuation values in and/or near conventionally unclassifiable regions such as metal, air pockets, bones, and lungs, thereby allowing for accurate PET quantitation and image reconstruction.